Method and device for variably setting the brake force in a hydraulic brake system of a motor vehicle

ABSTRACT

A method for variably setting the braking force in a hydraulic brake system of a motor vehicle with a brake pressure is produced which is applied to a wheel-braking device. In order to compensate a diminishing braking effect in hydraulic brake systems in wide ranges, the real actual friction factor between the brake disc and brake pad of at least one wheel brake is determined and compared with a prescribable desired friction factor, in the event of an impermissible undershooting of the actual friction factor by comparison with the desired friction factor, the brake pressure or a variable correlated with the brake pressure being increased to a value multiplied by a correction factor if the actual friction factor is within a defined friction factor stabilization range which comprises a plurality of friction factors below the desired friction factor.

The invention relates to a method and a device for variably setting thebraking force in a hydraulic brake system of a motor vehicle accordingto the preamble of claims 1 and 15, respectively.

DE 35 26 556 A1 discloses a brake system for motor vehicles whichcomprises a master cylinder which is designed as a tandem cylinder andis actuated by a brake pedal. Also provided is an auxiliary pressuresupply system which comprises a hydraulic pump for producing therequired brake pressure, and an electric motor actuating the pump, thehydraulic medium delivered by the pump being fed to the wheel brakingdevice in order to produce the required braking force.

Printed publication DE 44 27 170 C1 discloses a method for ensuring thebraking effect of brakes of vehicles in the event of moisture. For thispurpose, a conclusion is drawn on the presence of moisture via a sensordevice. If moisture is present, the time period is determined duringwhich the brakes of the vehicle have not been actuated. If this timeperiod overshoots a time threshold value, a braking operation isundertaken in which a vehicle retardation not perceptible to the driveris initiated. This braking operation vaporizes the water beingprecipitated on the brake and, the immediate and complete functionalityof the brake is ensured. Moreover, DE 44 27 170 C1 discloses detectingthe temperature of the brake parts and undertaking a braking operationfor the case that the brake temperature undershoots a temperaturelimiting value. There is no disclosure of a determination of thefriction factor of the brake disc/brake pad pairing nor of acompensation, based thereon, of a diminishing friction factor by meansof increasing the brake pressure.

A further device for measuring and/or regulating the braking force inthe brake system of a motor vehicle is disclosed in EP 0 189 082 A2. Itis proposed in this printed publication to set the braking force of thewheel brakes so as not to overshoot a permissible maximum temperaturevalue. This is intended to permit functioning continuous operation witha high, constant braking force. However, this device has thedisadvantage that there is a need during the regulation to take accountof relatively long time delays in the propagation of heat in the brakesystem, as a result of which the brake system to be regulated reactsrelatively sluggishly, and maximum braking forces cannot be maintainedcontinuously.

The publication PATENT ABSTRACTS OF JAPAN vol. 017, No. 430 (M-1460)discloses a device with the aid of which there is determined for thebrake pressure a desired value which corresponds to the brake pressurewhich is to be set by the driver at the front wheels on the basis of theactuation of the brake pedal. The friction factor present for the brakedisc/brake pad pairing is taken into account when calculating thisdesired value. This friction factor is determined as a function of thetemperature and the rotational speed of the brake disc. No provision ismade to increase the brake pressure in the case of an impermissibleundershooting of the actual friction factor by comparison with thedesired friction factor.

GB-A-2 297 134 A relates to a method and a device for controlling anABS/PCS system. The aim in this case is to save entirely or partly onthe vacuum brake busting, and to implement hydraulically the necessarybrake busting by suitably driving a return pump and valves which arerespectively included in the hydraulic unit. In addition to theimplementation of pressure control in which the brake pressure in thewheel brake cylinders is set in accordance with the actuation of thebrake pedal undertake by the driver, a function is also described withthe aid of which a [inaudible] of the friction factor between the brakepads and brake discs, which is denoted as fading, is detected andappropriate counter measures are taken. Various evaluations areundertaken in order to be able to detect a diminution in the frictionfactor. Thus, a check is made as to whether the brake pressureprevailing in the master brake cylinder is higher than a thresholdvalue; as to whether there is an actuation of the stop light switch, asto whether the ABS regulation is active; as to whether the speed of thevehicle is greater than zero or a threshold value; and as to whether theABS system is operating properly. The pressure in the wheel brakecylinder is increased if a diminution in the friction factor isdetected. The pressure is preferably raised in steps until the ABSregulations sets in at all wheels of the vehicle, or a maximumpermissible prescribable pressure difference between the pressure of thewheel brake cylinder and the pressure in the master brake cylinder isreached. The determination of the actual friction factor of the brakedisc/brake pad [inaudible], and an evaluation of the same with the aidof a desired friction factor are not disclosed. Also not disclosed is toincrease the brake pressure to a value multiplied by a correction factorif the actual friction factor is within a defined friction factorstabilization range.

The following object emerges against this background: the aim is for adiminishing braking effect in hydraulic brake systems firstly to bedetected simply and reliably, and secondly to be compensated in wideranges without the brake system being overloaded in the process.

This object is achieved according to the invention with the aid of thefeatures of claims 1 and 15, respectively.

The actual friction factor between the brake disc and brake pad of awheel brake characterizes the conversion of the clamping force of thecalliper into the retarding force on the brake disc. If the actualfriction factor between the brake disc and brake pad impermissiblyundershoots the desired friction factor, the brake pressure is steppedup, as a result of which a diminished brake power, which can arise as aconsequence of overheating, wear or environmental influences such asmoisture, pollution or the formation of ice, can be at least partiallycompensated such that no worsening of the brake power occurs, eithersubjectively or objectively. In order to prevent over-stressing of thebrake system and brake fading resulting therefrom, stressing of thebrake system and brake fading resulting therefrom, or to prevent apossible brake failure as a consequence of the increase in brakepressure carried out automatically, as an additional condition beforethe brake pressure increase, a check is made as to whether the actualfriction factor is within a defined stabilization range which comprisesa band of friction factors below the desired friction factor marking amaximum. The increase in brake pressure is carried out if the actualfriction factor lies within this stabilization range. If the actualfriction factor lies below the stabilization range, for reasons ofoverload protection it is not expedient to carry out an increase inbrake pressure; in this case the brake pressure increase does not takeplace, and fault or alarm signal is displayed which informs the driverof the poor brake condition.

The stabilization range can be permanently prescribed, or it can bedetermined in the course of operation with the aid of changing statevariables or performance quantities such as, for example, maximumachievable retardation, temperature of the brake system, wetting of thebrake parts with moisture, etc., a high measure of flexibility beingachieved thereby.

In an advantageous development, the stabilization range is subdividedinto a constant range, in which a constant, maximum braking retardationcan be maintained over deteriorating friction factors, and into agradient range in which although the brake pressure is piled the maximumbraking retardation is not achieved. The constant range in this casecomprises higher friction factors than the gradient range and the tworanges together expediently fill up the stabilization range completely.The boundary between the constant range and gradient range—the lowerfriction factor of the constant range—is advantageously variably set.With increasing temperature of the wheel-braking device, it isparticularly expedient to displace the lower friction factor of theconstant range in the direction of the lower limit of the gradientrange, as a result of which the constant range is expanded at theexpense of the gradient range. The result of this displacement of thelower friction factor of the constant range is to achieve a constantretardation over an increasing range of small friction factors withoutdisplacing the lower limit, serving the functional reliability, of thestabilization range further in the direction of smaller frictionfactors.

In order to compensate diminishing braking forces, the brake pressure isoperated on by a correction factor which is greater than one and whichis expediently calculated as a function of the measured or computedactual friction value, the desired friction value and the lower limitingvalue of the current range—the gradient range or the constant range. Thecorrection factor advantageously rises linearly in this case in theconstant range, starting from the value 1, with decreasing frictionfactors, and subsequently drops in the gradient range with decreasingfriction factors in a continuous and linear fashion until it reaches thevalue 1 for the lower friction factor of the gradient range.

The correction factor becomes ever larger in the constant range, thesmaller the lower friction factor of the constant range becomes, that isto say the more the lower friction factor of the constant range isdisplaced in the direction of the lower limit of the gradient range. Itis thereby possible to take account of the fact that an increasing brakepressure is required with increasing temperature in order to compensatethe temperature-induced brake fading.

Further advantages and expedient embodiments are to be gathered from thefurther claims, the description of the figures and the drawings, inwhich:

FIG. 1 shows a diagram with the profile of the correction factor forincreasing the brake pressure as a function of the friction factor, and

FIG. 2 shows a diagram with the profile of a friction factor excessivelyincreased by the correction factor, as a function of the real actualfriction factor.

In accordance with FIG. 1, the friction factors μ, plotted on theabscissa, between the brake disc and brake pad of a wheel brake aresubdivided into different ranges. Extending between a lower limitingvalue μ_(min) and a desired friction factor μ_(soll), whichsimultaneously marks the usually reachable friction factor maximumbetween the brake disc and brake pad, is a stabilization range 1 withinwhich additional measures are taken to boost the braking effect of ahydraulic, in particular an electro-hydraulic vehicle brake. Thestabilization range 1 is subdivided into a lower gradient range 2 and anupper constant range 3, the gradient range 2 being between the lowerfriction factor μ_(min), marking the lower limit of the stabilizationrange 1, and the friction factor μ_(low), marking the lower limit of theconstant range 3, and the constant range 3 is between the frictionfactor μ_(low) and the desired friction factor μ_(soll). The gradientrange 2 and the constant range 3 border directly one another, bothregions 2 and 3 completely filling up the stabilization range 1.

The correction factor f_(korr) serves to compensate diminished frictionfactors in order to ensure a desired vehicle retardation even in thecase of worsened friction factors. The correction factor f_(korr) ismultiplied by the brake pressure determined by calculation in theelectro-hydraulic brake and corresponding to a specific pedal pressureof the driver, it being possible as a result to compensate a diminishingbraking effect. The correction factor f_(korr) assumes a value greaterthan one only within the stabilization range 1, while outside thestabilization range 1 the value of the correct factor is equal to onesuch that brake pressure piling is carried out only for actual frictionfactors μ_(ist) within the stabilization range 1, but not above theupper limit of the stabilization range 1, the desired friction factorμ_(soll), and below the lower limit of the stabilization range 1, thelower friction factor μ_(min).

The determination of the correction factor as a function of the realactual friction factor, and the multiplication of the correction factorby the brake pressure corresponding to the pedal pressure of the driverand determined in the braking device offer the advantage that it ispossible to react to a reduction in the actual friction factor directlyand without loss of time. The brake system can be kept at its original,constant braking level independently of the physical cause of the lossin friction factor, at least within the constant range 3.

Within the stabilization range 1, the value of the correction factorf_(korr) is greater than one, the function of the correction factorrising linearly, as a function of the friction factor for the lower orthe upper limiting value μ_(min), μ_(soll), starting at one, to thefriction factor μ_(min) lying between the gradient range 2 and constantrange 3. The function of the correction factor assumes a maximum in thecase of the friction factor μ_(min) and the function of the correctionfactor assumes a triangular form in the case of a linear rise.

The correction factor f_(korr) is calculated within the constant range 3as a function of the instantaneous actual friction factor μ_(ist) inaccordance with the linear relationship

f _(korr)=1+(μ_(soll)/μ_(low)−1)*(μ_(soll)−μ_(ist))/(μ_(soll)−μ_(low)),

and the correction factor f_(korr) is determined within the gradientrange 2 using the linear law

f _(korr)=1+(μ_(ist)−μ_(min))/(μ_(low)−μ_(min))*(μ_(soll)/μ_(low)−1).

Instead of a linear function, it can be expedient to prescribe anon-linear profile for the correction factor f_(korr) as a function ofthe actual friction factor μ_(ist).

The current actual friction factor μ_(ist) is advantageously determinedfrom the vehicle retardation as measured or determined by calculation,and the current brake pressure applied. However, it can also beexpedient to calculate the actual friction factor μ_(ist) from acomparison between the desired and actual vehicle retardation.

A minimum friction factor μ_(Alarm) is plotted below the lower limitingvalue μ_(min) of the stabilization range 1. If the current actualfriction factor μ_(ist) reaches the minimum friction factor μ_(Alarm),an error signal is generated and displayed to the driver as anindication of a brake defect or of a dangerous situation.

FIG. 2 shows a representation of the friction factor increased by thecorrection factor f_(korr), plotted against the real actual frictionfactor μ_(ist). The increasing of the friction factor μ_(ist) isperformed exclusively in the stabilization range 1, within which thefunction of the increased friction factor deviates from a straight linethrough the origin. In the gradient range 2, the function of thefriction factor μ_(ist) has a section 4, the gradient of which isincreased by comparison with the straight line through the origin,whereas in the constant range 3 the corresponding section 5 of thefunction of the friction factor μ_(ist) is, admittedly, above the levelof the straight line through the origin, but at a level which remainsconstant. As a consequence of the multiplication by the correctionfactor f_(korr), a decreasing actual friction factor μ_(ist) iscompensated in the constant range 3 to such an extent that a constantbraking retardation is achieved despite a reduced actual frictionfactor. The correction factor f_(korr) can thereby either, asillustrated, be used to multiply the determined actual friction factorμ_(ist), which features in the calculation of the required hydraulicbrake pressure, or be used directly to multiply the brake pressure or tomultiply some other variable which influences the brake pressure or thevehicle retardation.

The device for variably setting the braking force in the brake system ofa motor vehicle comprises a regulating and control unit in which theactuating signals required for setting the desired brake pressure aregenerated. For this purpose, the regulating and control unit is fed theactual friction factor μ_(ist) as input signal, or the actual frictionfactor μ_(ist) is calculated from measured variables and compared with adesired friction factor μ_(soll). If the actual friction factor μ_(ist)undershoots the desired friction factor μ_(soll) in an impermissibleway, and if, moreover, the actual friction factor μ_(ist) is within thestabilization range, the regulating and control unit generates anactuating signal with the aid of which the brake pressure is increasedto a value multiplied by the correction factor f_(korr).

In an expedient development, it can be provided for the purpose ofimproving the braking performance that in the event of moisture a lowbrake pressure at a level, advantageously, of approximately 3 bar to 8bar is built up even during unbraked driving, in order to expel a waterfilm on the brake disc by the application of the brake pads to the brakediscs. The low brake pressure ensures that no perceptible, undesiredvehicle retardation occurs. In order to avoid unnecessary automaticinstances of brake actuation in the event of moisture, it can beexpedient to trigger the braking function via a rain sensor and/or theuse of the windscreen wiper. If appropriate, the engine torque isincreased, for example by increasing the quantity of fuel injected andthe corresponding air feed, in order to compensate the braking torque,and to keep the vehicle speed constant.

Furthermore, the risk of corrosion in the brake system can be reduced bybuilding up a low brake pressure without perceptible vehicleretardation, as a result of which the temperature of the brake systemcan be stabilized at a low level harmless to the brake material, even inthe case of unbraked driving. As a consequence of the increased braketemperature, the moisture is largely removed or kept away from the wheelbrake. As an additional effect, the brake disc is freed from dirt andthawing salt.

The build-up of the brake pressure during unbraked driving can becarried out at intervals, the length of the intervals being determinedby the length of driving times without braking triggered by the driver,by the wiper setting and by the braking load of a preceding brakingoperation.

In order to prevent vapour lock in the brake fluid as a consequence ofbrake overheating, the boiling point of the brake fluid can be increasedby specifically including a residual pressure in the brake system, thuspreventing the fluid from gassing out. Particularly in the case of veryfrequent and/or harsh actuation of the brakes in conjunction with slightactuation of the accelerator pedal, there is a risk of overheating inthe brake system, and this can be counteracted by inclusion of theresidual pressure. As a further parameter, the operating time of thebrake fluid since the last change of brake fluid can be taken intoaccount. It is expedient for the driver to be advised of theoverstressing of the brake system.

What is claimed is:
 1. A method for variably setting the braking forcein a hydraulic brake system of a motor vehicle when a brake pressure isproduced which is applied to a wheel-braking device, comprising thesteps of: determining a real actual friction factor (μ_(ist)) between abrake disc and brake pad of at least one wheel brake and comparing anactual friction factor with a prescribable desired friction factor(μ_(soll)), and increasing the brake pressure or a variable correlatedwith the brake pressure to a value multiplied by a correction factor(f_(korr)) when an impermissible undershooting of the actual frictionfactor (μ_(ist)) by comparison with the desired friction factor(μ_(soll)) occurs, and if the actual friction factor (μ_(ist)) is withina defined friction factor stabilization range below the desired frictionfactor (μ_(soll)), the correction factor (μ_(korr)) being set to a valuegreater than one.
 2. The method according to claim 1, wherein thefriction factor stabilization range is subdivided into a lower gradientrange and an upper constant range wherein a constant braking retardationin the constant range is achieved.
 3. The method according to claim 2,wherein the correction factor (f_(korr)) rises within the constant rangewith decreasing actual friction factor (μ_(ist)).
 4. The methodaccording to claim 3, wherein the correction factor (f_(korr)) riseslinearly.
 5. The method according to claim 2, wherein in the constantrange, the correction factor (f_(korr)) obeys the law f_(korr)=1+(μ_(soll)/μ_(low)−1)*(μ_(soll)−μ_(ist))/(μ_(soll)−μ_(low))wherein μ_(ist) denotes the actual friction factor, μ_(soll) denotes thedesired friction factor, and μ_(low) denotes the lower friction factorof the constant range.
 6. The method according to claim 2, wherein inthe gradient range, the correction factor (f_(korr)) obeys the law f_(korr)=1+(μ_(ist)−μ_(min))/(μ_(low)−μ_(min))*(μ_(soll)/μ_(low)−1)wherein μ_(min) denotes the lower friction factor of the gradient range.7. The method according to claim 1, wherein a minimum friction factor(μ_(Alarm)) lying below the stabilization range is defined, and an errorsignal is generated upon undershooting of the minimum friction factor(μ_(Alarm)).
 8. The method according to claim 2, wherein the lowerfriction factor (μ_(min)) of the gradient range, the lower frictionfactor (μ_(low)) of the constant range and/or the desired frictionfactor (μ_(soll)) can be prescribed as a function of vehicle performancequantities, state variables or parameters.
 9. The method according toclaim 8, wherein with increasing temperature of the wheel-braking devicethe lower friction factor (μ_(low)) of the constant range is displacedin the direction of the lower friction factor (μ_(min)) of the gradientrange.
 10. The method according to claim 1, wherein the actual frictionfactor (μ_(ist)) is determined from the measured vehicle retardation andthe current brake pressure.
 11. The method according to claim 1, whereinin the event of moisture, the brake pads are applied with low brakepressure to the brake discs.
 12. The method according to claim 11,wherein the application of the brake pads to the brake disc is triggeredby a signal from a rain sensor.
 13. The method according to claim 11,wherein the application of the brake pads to the brake disc is triggeredby the start-up of the wiper.
 14. The method according to claim 1,wherein the brake pads are applied to the brake disc to such an extentthat the temperature of the wheel-braking device approximately assumes adesired temperature.
 15. A device for variably setting the braking forcein a hydraulic brake system of a motor vehicle, having a regulating andcontrol unit for generating actuating signals for setting a brakepressure in a wheel-braking device, wherein a real actual frictionfactor (μ_(ist)) between a brake disc and brake pad of at least onewheel brake is fed as an input signal to the regulating and control unitor an actual friction factor is calculated in the regulating and controlunit from measured variables, and in that the actual friction factor iscompared with a prescribable desired friction factor (μ_(soll)), and inthat in the event of an impermissible undershooting of the actualfriction factor (μ_(ist)) by comparison with the friction factor(μ_(soll)), an actuating signal is generated in the regulating andcontrol unit and is used to increase the brake pressure to a valuemultiplied by a correction factor when the actual friction factor(μ_(ist)) is within a defined friction factor stabilization range belowthe desired friction factor (μ_(soll)).
 16. The method according toclaim 15, wherein the friction factor stabilization range is subdividedinto a lower gradient range and an upper constant range wherein aconstant braking retardation in the constant range is achieved.
 17. Themethod according to claim 16, wherein the correction factor (f_(korr))rises within the constant range with decreasing actual friction factor(μ_(ist)).
 18. The method according to claim 17, wherein the correctionfactor (f_(korr)) rises linearly.
 19. The method according to claim 16,wherein in the constant range, the correction factor (f_(korr)) obeysthe law f_(korr)=1+(μ_(soll)/μ_(low)−1)*(μ_(soll)−μ_(ist))/(μ_(soll)−μ_(low))wherein μ_(ist) denotes the actual friction factor, μ_(soll) denotes thedesired friction factor, and μ_(low) denotes the lower friction factorof the constant range.